1. The Field of the Invention
The present invention generally relates to an apparatus and system for removing excess heat generated in electrical components. More particularly, the present invention relates to an apparatus and method for removing heat from a magnetically suspended rotating anode inside an x-ray tube using liquid metals, the x-ray tube being for use in systems such as CT scanners and the like.
2. The Prior State of the Art
In systems utilizing electrical components, heat is often generated when electrical power is supplied to the component. With some components, the amount of heat generated can be substantial. In such an environment, the dissipated heat must be continuously removed so as to prevent overheating and damage to the components and/or surrounding electrical circuitry. One example of an electrical system in which overheating can be problematic is in systems utilizing high power x-ray tubes for commercial or medical applications. Such tubes are commonly found in various devices, for instance in CT (computerized tomographic) scanning, x-ray lithography for producing integrated circuits, x-ray diffraction for analyzing materials, x-ray detection devices for inspecting luggage, and the like. In such devices and applications, a high power, high intensity x-ray tube is used to produce the radiation. By way of example, a cooled X-ray tube is disclosed in U.S. Pat. No. 5,541,974, issued to Anderson et al. on Jul. 30, 1996, which is incorporated herein by reference.
Depending on the particular application, the radiation can be used in various ways. For instance, in a CT scanner, the radiation can be detected with one or more detectors after it has passed through the region of interest of a patient's body. It is then manipulated to reproduce an image of the anatomical region of interest. In the course of such a procedure, much heat is generated by the x-ray tube as a by-product of the x-ray energy generated. X-rays are emitted from a small portion of the track designate as the focal spot which is bombarded by electrons. The track is situated upon an anode that is rotated within a vacuum tube. The anode is rotated so that cool track material is passed into the focal spot region and heated. The hot material in the focal spot area is likewise moved out of this region to cool. Even though the heat is directed over a relatively large surface area on the track, anodes of high power tubes of this type frequently are heated sufficiently to become incandescent in response to the electron bombardments. X-ray tube anodes typically operate in a range from about zero to 1200.degree. C. At 1200.degree. C., an anode can be visibly red or yellow and readily radiates heat. The heat generated on the anode must be continuously removed to prevent damage to the tube (and any other adjacent components) and to increase the x-ray tube's overall service life. Moreover, applications using x-ray tubes experience the problem of downtime as the user waits for the tube to cool down to useable levels.
A high intensity x-ray anode can transfer heat through radiation from its hot surface or by conduction through connecting materials. Until recently, conventional rotating anodes in high intensity x-ray tubes have only been able to transfer heat through radiation. While radiative cooling transfers heat quickly when the anode is extremely hot (e.g. 1200.degree. C.), the rate of heat transfer drops quickly as the anode cools to temperatures levels which are still too high for efficient operation or generation of x-rays. At high temperatures, such as 1200.degree. C., some materials from which the anode is made can experience problems. The high temperatures can cause shifting of portions of the anode, cracking, distressing, warping, and other material failures. Material failures can result in errors in the resultant x-ray image.
Heat from the rotating anode can be radiated or conducted to the bearing on which the anode rotates. The bearings can be ball bearings. Cyclical heating and cooling of the ball bearings shortens their life, and creates the further problems of unacceptably high noise levels and vibration.
To avoid the foregoing problems known to exist in using ball bearings in which to journal the rotating shaft of an anode, the anode can be magnetically suspended within the vacuum tube and rotated during the x-ray generation process. Using magnetic suspension of the anode within the vacuum tube also can experience problems with heat. Heat from the rotating anode can be radiated or conducted to the bearing on which the anode rotates. Materials that are used to magnetically suspend the anode can be heated near to or above their Curie point so that the materials cease to properly function. The magnetic effect cannot be maintained above the Curie temperature of the materials. Past applications used to cool such materials dealt only with cooling by radiation. As such, these applications have a limitation in terms of their operating parameters in that they cannot properly cool at temperatures below about 500.degree. C. which is near the Curie point of most materials used to generate the suspending magnetic field. As such, x-ray tubes of this variety may not be suitable for use in connection with certain types of CT devices or devices wherein the heat generated by the tube may exceed 500.degree. C.
When it is desirable to operate an x-ray tube at a very high temperature, e.g. above 1000.degree. C., heat is radiated from the anode. To radiate effectively so as to remove the radiated heat, it is desirable to coat the inside of the vacuum tube and other parts therein with an emissive coating. The emissive coating enhances thermal heat radiation properties. At these temperatures emissive coatings are used on the anode and on the vacuum envelope to facilitate the removal of waste heat. The coating, when subjected to high temperatures and large temperature fluctuations, peels, cracks, and flakes and generally releases particles into the tube. These particles cause arcs. An arc is an uncontrolled rush of electrons hitting the target or some other element. The arc causes a disruption in the x-ray production process and in the intended application.
Arcs can also be caused by outgassing within the tube. When a material is heated gas can be evolved from the material. Gas is evolved faster at successively higher temperatures. This is known as outgassing. Accordingly, it is desirable to use materials in the tube that do not outgas at operating temperatures, or to operate the x-ray tube below a temperature at which materials in the tube tend to outgas.
One way of dissipating heat in x-ray tubes is with a liquid coolant or fluid, such as a dielectric oil. In a cooling system of this sort, the x-ray tube is usually disposed within an x-ray tube housing which is filled with coolant. A pump is used to continuously circulate the coolant through the housing. Then, as heat is dissipated by the x-ray tube, at least some of it is absorbed by the coolant fluid. The heated coolant fluid is then passed to some form of heat exchange device, such as a radiative surface in the form of a heat exchanger. Conventional heat exchangers include fins that are air-cooled by air blowing across the fins. The fluid is then re-circulated by the pump back into the x-ray tube housing and the process repeated.
Another way to remove heat is to blow air directly on the surface of the housing which may have its surface area enhanced with the addition of fins.
Yet another way to remove heat is to blow air directly on the surface of the vacuum tube and forego the use of coolant inside the housing entirely.
It is difficult to remove heat from the anode by conduction when it is rotating. This can be done, however, by putting the rotating anode in contact with liquid metal. In this technique, heat is removed from the anode by the placement of a relatively high thermal conductive liquid metal in the thermal pathway between the rotating anode and a stationary heat removing structure. The liquid metal is usually gallium, indium, tin, or a gallium-indium-tin alloy. Gallium is used because it has a sufficiently low vapor pressure to be compatible with the low pressures within the vacuum tube. Prior art liquid metal cooling apparatus fail to adequately cool the anode in high energy applications in that they lack a high conductivity heat path from the track surface of the anode, through the liquid metal, and then to a high conductivity heat exchanger.